1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device using multiplexers capable of providing an improvement of line mura between pixels.
2. Description of Prior Art
More and more advanced-function displays have found their applications in current consumer electronic products. In particular, liquid crystal displays (LCDs) having a high-resolution color screen, are more widely applied in various electronic devices, such as televisions, mobile phones, personal digital assistances (PDA), digital cameras, desktop computer screens, and notebook computer screens.
Referring to FIG. 1 showing a functional block diagram of a prior art liquid crystal display device 10, the conventional liquid crystal display device 10 contains a liquid crystal panel 12, a gate driver 14, and a source driver 16. The liquid crystal panel 12 includes a plurality of pixels, each pixel having three pixel units 20 indicating three primary colors, red, green, and blue. For example, the liquid crystal display 12 with 1024 by 768 pixels contains 1024×768×3 pixel units 20. The gate driver 14 periodically outputs a scanning signal to turn on each transistor of the pixel units 20 row by row, meanwhile, each pixel units 20 is charged to a corresponding voltage based on a data signal from the source driver 16, to show various gray levels. After a row of pixel units is finished to be charged, the gate driver 14 stops outputting the scanning signal to this row, and then outputs the scanning signal to turn on the transistors of the pixel units of the next row. Sequentially, until all pixel units 20 of the liquid crystal panel 12 finish charging, and the gate driver 14 outputs the scanning signal to the first row again and repeats the above-mentioned mechanism.
As to the conventional liquid crystal display, the gate driver 14 functions as a shift register. In other words, the gate driver 16 outputs a scanning signal to the liquid crystal display 12 at a fixed interval. For instance, a liquid crystal display 12 with 1024×768 pixels and its operating frequency with 60 Hz is provided, the display interval of each frame is about 16.67 ms (i.e., 1/60 second), such that an interval between two scanning signals applied on two row adjacent lines is about 21.7 μs (i.e., 16.67 ms/768). The pixel units 20 are charged and discharged by data voltage from the source driver 16 to show corresponding gray levels in the time period of 21.7 μs accordingly.
In general, as a skilled person in this art is aware, the voltage across the two electrodes has two polarities. A voltage of the pixel electrode larger than that of the common electrode is called positive polarity, and inversely, a voltage of the common electrode larger than that of the pixel electrode is called negative polarity. If absolute values of the voltage difference across the two electrodes are identical, no matter whether the voltage value of the pixel electrode or that of the common electrode is higher, an identical gray level is obtained. However, in fact, opposed voltage difference value across the two electrodes results in the opposed alignments of the liquid crystal molecules.
From a view of long-term sum effect, if the voltage across the two electrodes tends toward either polarity for a while, the common voltage applied on the common electrode causes a voltage-drifting phenomenon. Consequently, the alignment of the liquid crystal molecules fails to be varied based on the required control voltage, resulting in displaying incorrect gray levels. In an extreme situation, it is possible that if the voltage across the two electrodes tends toward either polarity for a long while, even if no voltage is applied, the liquid crystal molecules still fail to be aligned based on a variety of electrical fields. As a result, in order to prevent the common voltage from experiencing the voltage-drifting phenomenon as the voltage applied across the two electrodes tends toward any polarity, the voltages across the two electrodes are periodically switched between positive polarity and negative polarity. Dot inversion mode, by which each pixel unit alternately varies polarity and its neighbor pixels have inversed polarities all the time, is widely used for periodically driving the voltage across the liquid crystal molecules.
With reference to FIG. 1 showing an LCD panel according to prior art, and FIG. 2 illustrating a schematic diagram of the LCD panel using dot inversion mode alternately varying polarity, the LCD panel 12 comprises a plurality of first pixel groups and a plurality of second pixel groups. Each first pixel group comprises six pixel units R11, R12, G11, G12, B11, and B12; each second pixel group comprises six pixel units R21, R22, G21, G22, B21, and B22. The pixels R11, R12, R21, R22 are indicated to show red, the pixels G11, G12, G21, G22 are indicated to show green, and the pixels B11, B12, B21, B22 are indicated to show blue. Each first pixel group is coupled to a first polarity data voltage S1 via a first multiplexer 26, while each second pixel group is coupled to a second polarity data voltage S2 via a second multiplexer 28. Herein, the conventional LCD panel having 1 to 6 multiplexers configuration may improve display quality and reduce a number of output pins of source drivers to lower cost.
Considering the first pixel group, switch units SW11, SW12, SW13, SW14, SW15, and SW16 are coupled to pixel unit R11, G11, B11, R12, G12, and B12, respectively. Considering the second pixel group, switch units SW21, SW22, SW23, SW24, SW25, and SW26 are coupled to pixel R21, G21, B21, R22, G22, B22, respectively. The switch units SW11, SW21 are controlled by scan signal voltage SCAN1. The switch units SW12, SW22 are controlled by scan signal voltage SCAN2. The switch units SW13, SW23 are controlled by scan signal voltage SCAN3. The switch units SW14, SW24 are controlled by scan signal voltage SCAN4. The switch units SW15, SW25 are controlled by scan signal voltage SCAN5. The switch units SW16, SW26 are controlled by scan signal voltage SCAN6.
During an interval t1, when the switch unit SW11, SW21 turns on in response to the scan signal voltage SCAN1, the pixel unit R11 of the first pixel group, and the pixel unit R21 of the second pixel group shows corresponding grey levels based on the polarity signal voltage S1, S2, respectively. During an interval t2, when the switch unit SWT2, SW22 turns on in response to the scan signal voltage SCAN2, the pixel unit G11 of the first pixel group, and the pixel unit G21 of the second pixel group shows corresponding grey levels based on the polarity signal voltage S1, S2, respectively. During an interval t3, when the switch unit SW13, SW23 turns on in response to the scan signal voltage SCAN3, the pixel unit B11 of the first pixel group, and the pixel unit B21 of the second pixel group shows corresponding grey levels based on the polarity signal voltage S1, S2, respectively. During an interval t4, when the switch unit SW14, SW24 turns on in response to the scan signal voltage SCAN4, the pixel unit R12 of the first pixel group, and the pixel unit R22 of the second pixel group shows corresponding grey levels based on the polarity signal voltage S1, S2, respectively. During an interval t5, when the switch unit SW15, SW25 turns on in response to the scan signal voltage SCAN5, the pixel unit G12 of the first pixel group, and the pixel unit G22 of the second pixel group shows corresponding grey levels based on the polarity signal voltage S1, S2, respectively. During an interval t6, when the switch unit SW16, SW26 turns on in response to the scan signal voltage SCAN6, the pixel unit B12 of the first pixel group, and the pixel unit B22 of the second pixel group shows corresponding grey levels based on the polarity signal voltage S1, S2, respectively.
Nevertheless, as shown in FIG. 2, under dot inversion mode incorporating with 1 to 6 multiplexer scheme, the pixel units B12 and R21 always have identical polarity, leading a junction between the pixel units B12 and R21 to a failure of dot inversion, and thereby line mura effect across the pixel units B12 and R21.